The Pulpit Commentary
By the rivers of Babylon The Euphrates and the canals derived from it, which were many, and filled with running, not stagnant, water. These would present themselves to the exiles as "rivers." There we sat down, yea, we wept, when we remembered Zion. The exiles had their leisure hours—they were not kept by their masters at hard work continually. During these leisure hours they naturally "sat down" by the rivers of Babylon, as the most pleasant and attractive places. They brought their harps with them (Psa_137:2), with some idea, perhaps, of indulging in mournful strains. Grief, however, overpowered them—Zion came to their recollection-and they could do nothing but weep.

The Pulpit Commentary
We hanged our harps upon the willows in the midst thereof. The superfluous "harps" were "hung" up upon the trees that grew by the watercourses. These are called "willows," or, according to some, "poplars," but were probably of a different species from any of the trees that grew in Palestine. The chief Babylonian tree was the palm, which grew in the greatest luxuriance along the courses of all the streams. Tamarisks, poplars, and acacias were also common, but true "willows" hardly appear to have ever been a product of the country. The 'arabah of our author was probably either a poplar or a tamarisk.

The Pulpit Commentary
For there they that carried us away captive required of us a song; literally, words of song. The oppressors break into the retired gathering of their captives, and "require of them a song"—demand roughly and rudely to be entertained with the foreign music, which is perhaps sweeter than their own, or at any rate more of a novelty. And they that wasted us required us mirth. Not only was "a song" wanted but a joyous song - one that would wake feelings of mirth and gladness in those who heard it. Saying, sing us one of the songs of Zion; literally, sing us frown a song of Zion. The captives had, no doubt, spoken of the joyous strains which they had been wont to pour forth in their own city upon festive occasions. Their conquerors demand a specimen, but are repulsed with the words of the next verse.

The Pulpit Commentary
How shall we sing the Lord's song in a strange land? The "songs of Zion" are Jehovah's songs, used in his worship, suited only for religious occasions. It would be desecration to sing them "in a strange land," among strange people, not to call forth devotional sentiment, but to gratify curiosity.

The Pulpit Commentary
If I forget thee, O Jerusalem, let my right hand forget her cunning; literally, let my right hand forget; but the words supplied in the Authorized Version are necessary to bring out the sense, which is, "If I forget thee, O Jerusalem, so far as to desecrate thy sacred songs by making them an entertainment for the heathen, may I never have power to strike a note again!"

The Pulpit Commentary
If I do not remember thee, let my tongue cleave to the roof of my mouth. Let me be deprived of the power of song. What was wished in the preceding verso with respect to the power of instrumental performance is here wished with respect to the vocal organs. If I prefer not Jerusalem above my chief joy. This seems to be the true sense, and is equivalent to "If I prefer not Jerusalem above aught else."

The Pulpit Commentary
Remember, O Lord, the children of Edom in the day of Jerusalem; rather, remember, O Lord, to the children of Edom the day of Jerusalem. "The day of Jerusalem" is the day of her fall, when Edom took part with her enemies, and rejoiced at her destruction (see Lam_4:21, Lam_4:22; Eze_25:12; Eze_35:5; Oba_1:10-14). The psalmist prays God to "remember" this to Edom, and requite it upon her (comp. Psa_132:1, where the same expression is used in a good sense). Who said, Rase it, rase it, even to the foundation thereof; i.e. "destroy the city utterly—leave not one stone upon another." The enmity between Edom and Israel was of the intensest character (see 1Ki_11:15, 1Ki_11:16; 1Ch_18:12; Jer_49:7-22; Amo_1:11, Amo_1:12; Mal_1:3-5).

The Pulpit Commentary
O daughter of Babylon; i.e. O nation of the Babylonians (comp. Isa_47:1, Isa_47:5; Psa_9:14, etc.). Who art to be destroyed; literally, thou desolated one. The desolation of Babylon began with its capture by Cyrus, but was not completed for many centuries. In the Archaemenian period it was one of the chief cities of the empire. Even under the Parthians it was still a flourishing town. But from the time of Isaiah's prophecy (Isa_13:1-22) it was a doomed city, and in the eyes of a devout Jew already "desolate." Happy shall he be, that rewardeth thee as thou hast served us; i.e. happy shall he be that completes thy destruction, and the destruction of thy people. He will be the instrument for carrying out God's vengeance.

The Pulpit Commentary
Happy shall he be, that taketh and dasheth thy little ones against the stones; i.e. that brings on thee the worst calamities of war (see 2Ki_8:12; Isa_12:1-6 :16-18; Hos_10:14; Hos_13:16; Nah_3:10).